The invention relates generally to safety systems that limit the amount of radiant energy that can be emitting from an open optical fiber or a transmitter port in an optical communication link. More particularly, the invention relates to a safety system for automatically propagating shutting down and restoring of links in a multi-link fiberoptic connection employing the Open Fiber Control safety standard, wherein the automatic shutting down and/or restoring of the links is in response to opening or closing of one or more of the links in the connection.
Communication between computer systems usually involves a sending system (sender) sending a command to a receiving system (receiver) over a link used to couple the sender and the receiver. The receiving system then, typically, sends a response back over the link to the sending system.
One example of a link used by International Business Machines Corporation to communicate between a sender and a receiver is an intersystem channel (ISC) link. In particular, an intersystem channel link couples an intersystem channel adapter on one system (e.g., a central processor) that may be used for sending or receiving messages with an intersystem channel adapter on another system (e.g., a coupling facility that contains data shared by any central processor coupled thereto) that may also be used for sending or receiving messages.
An ISC link is a fiberoptic link which employs the laser safety feature entitled xe2x80x9cOpen Fiber Controlxe2x80x9d (OFC). One implementation of an OFC module is described in U.S. Pat. No. 5,136,410, entitled xe2x80x9cOptical Fiber Link Control Safety Systemxe2x80x9d, which is hereby incorporated herein by reference in its entirety. The OFC standard requires that in order for fiberoptic transceivers at each end of a link to function, there must be a closed link between the duplex transmitter/receiver pairs at the ends of the link. If, for example, a link is opened due to a pulled connection or broken fiber, then the hardware must automatically shut off the transmitters at both ends of the link. At this point, the standard dictates that the transmitters will send out a short optical pulse, for example, every ten seconds, trying to reestablish the link. When the fiber is reconnected, the link will automatically resume operation pursuant to receipt of these short optical pulses.
In order to extend the distance of an ISC link from, for example, 3 kilometers to over 20 kilometers, it is desirable to insert repeaters into the ISC link. Unfortunately, a conventional optical repeater cannot adequately propagate a loss of light signal. The American National Standards Institute (ANSI) transceiver implementation, and the OFC timing requirements, make it impossible to directly propagate loss of light across the optical repeater. The OFC timing requirements are part of the ANSI Fiber Channel Standard itself; e.g., reference the American National Standards Institute (ANSI) Fiber Channel Physical and Signaling Interface (FC-PH), document no. X3.230 (1997), available through Global Engineering Documents of Santa Anna, Calif. Thus, if a first link of a multi-link connection is opened, the other links of the connection will not drop light (and vice versa). This causes problems for the system, including a serious data integrity issue because the invalidate bit at the devices coupled to the multi-link connection will not be set properly unless the other links of the connection also show loss of light.
In view of the above, a need exists in the art for an optical repeater capable of extending the practical length of a communication link which employs Open Fiber Control (OFC). More particularly, a method to artificially propagate loss of light over an extended ISC link, or any data link using Open Fiber Control per the Fiber Channel Standard, is needed.
Briefly summarized, this invention comprises in one aspect a method for propagating a link condition within a multi-link fiberoptic connection, wherein the link condition occurs on at least one link of the multi-link connection. The method includes: detecting an open-link condition on one link of the multi-link fiberoptic connection; and automatically propagating the open-link condition between links of the multi-link fiberoptic connection in accordance with Open Fiber Control specified in the American National Standards Institute Fiber Channel Standard to shut down at least some optical transmitters within the multi-link fiberoptic connection.
Those skilled in the art will note from the description provided herein that there are numerous advantages to the techniques of the present invention. For example, this invention allows all devices, processors, and repeaters attached to a communication channel with multiple link segments to be aware when a link segment opens and physical connectivity is lost. Propagation of the open-link condition complies with industry standard timings for Open Fiber Control, as specified by the ANSI Fiber Channel Standard. This means that a repeater in accordance with the present invention will interoperate with any device built by multiple vendors in the industry today. Another advantage is that the invention can be implemented in hardware only, which makes the processing time faster and simplifies the implementation. Further, implementation of the invention allows OFC propagation without any changes to hardware of the attached devices at the end of the multi-link connection. The only changes required are to the repeaters, at least in the hybrid processing approach disclosed herein, so the devices can interoperate with systems already in the field.
A further advantage is that the invention prevents deadlock conditions in which one or more links do not automatically restore themselves because different repeaters are continually signaling each other to turn off link segments. Further, this invention automatically restores physical connectivity when any open-link segment is closed again. The invention applies to both standard repeaters, i.e., single input/output and to multiplexers, i.e., multiple inputs/single output, for any link which uses OFC at any data rate. Also, the invention can be extended to any number of repeaters in a daisy chain arrangement.
As a still further advantage, the invention does not require that all transceivers in the link be equipped with electrical wrap capability if the attached device hardware can be modified to recognize an out band signal propagation. Further, propagation of the OFC open-link condition does not require a separate wire, fiber or communications path between adjacent repeaters and attached devices. All communication is carried out over the existing duplex fiberoptic links. The invention does not violate existing laser safety certifications on the attached devices or repeaters. Another advantage is that the invention enables the construction of repeaters for OFC links, therefore it allows construction of parallel computer processor systems at extended distances wherein fiberoptic links with OFC are relied upon to interconnect the multiple processors. By being able to move the processors in such a parallel system longer distances from each other, the possibility for disaster recovery is enhanced.
As another advantage, by applying the invention to multiplexers, it becomes possible to construct parallel computer process systems at extended distances without requiring large numbers of optical fibers between the locations. Use of multiplexers can reduce the number of inter-site links by combining many data channels over one physical link. Reducing the number of inter-site links both simplifies the installation and greatly reduces the cost of installing fiber.
Another advantage of the invention is that by specifying an out band signal as disclosed herein, both disparity balance and DC balance are maintained. This keeps the optical receivers from drifting out of specification during a loss of light condition, so that the link can reinitialize quickly when connectivity is restored.
A still further advantage of the invention is that no special data characters or sequences are needed to convey loss of light state across a link, so it is not necessary to modify software running over the link to reserve special control sequences or characters for the condition.